No John, it's not just some figure drawn out of a hat.
Following the usual consultations processes there was a review made by MAC (Migration Advisory Committee)
It went under the heading "Review of the Income Requirement for Sponsorship under the Family Migration Route"
If you have an interest here's a link to the document:-
Review of the Income Requirement for Sponsorship under the Family Migration Route
It's quite a long read. Over 80 pages.